This invention relates to a single-sideband radiotelephone system including at least one single-sideband transmitter and a plurality of single-side-band receivers.
As is well known, a single-sideband transmitter is designed to broadcast amplitude modulated information on a selected one of the two primary sidebands of an amplitude modulated carrier while suppressing the other sideband as well as the carrier. Since no carrier exists in the transmitted signals, a reinserted carrier is provided by a receiver, and it is with reference to this reinserted carrier that the sideband information is demodulated. The quality and frequency response of the demodulated signal depends on how accurately the frequency of the reinserted carrier is aligned with reference to the original, but now suppressed carrier. Therefore, determination of the proper insertion carrier frequency is essential if a single-sideband signal is to be demodulated accurately.
Usually, the stability of transmitters and the receivers is high enough to provide reasonably acceptable quality of voice communications. Some receivers are provided with a "clarifier" control which permits the operator to make minor adjustments in the insertion carrier frequency should the transmitter and receiver frequencies differ slightly.
There are several prior art systems which attempt to establish the proper insertion carrier frequency, one of which is shown in U.S. Pat. No. 3,716,790. In the device shown in that patent, a two-frequency tone is broadcasted continuously. For example, a F1 frequency tone is broadcast simultaneously with a selected one of an F2-F6 frequency tone. At the receiver, these two tones interact in the intermediate frequency (IF) and audio stages to produce a single audio tone having a frequency equal to the frequency difference between the two transmitted tones. The detected audio tone is independent of the frequency of the insertion carrier, and may be used as a guide in tuning the receiver. This system is wasteful of power and bandwidth in that the two frequency tones must be broadcast simultaneously during transmission of any information but do not carry any information other than to establish the proper insertion carrier frequency.
In a radiotelephone system, as well as in other systems, such as a radioteletype, a preselected receiver is accessed by means of tone coded signals which precede the broadcast of audio information. It is essential that these tone coded signals be detected and decoded properly if the preselected receiver is to be activated. In order to detect the tone coded signals, however, the detectors or filters used in the receiver will have a limited band-pass in order to provide sufficient selectivity. Therefore, the receivers must be tuned accurately in order to place the tone coded signals within the band-pass of the detectors or filters.
It is recognized, however, that within the frequency bands normally used by the radiotelephone service, frequency stability of .+-.0.01% for the transmitter is acceptable. Assuming a 2 MHz carrier signal, the carrier may therefore vary in frequency .+-.200 Hz. A similar variation in the frequency stability of the receiver could result in a total variation in the received signals of .+-.400 Hz. By way of example, if a single 1300 Hz audio tone were to be broadcasted, this tone could be demodulated at a receiver as a tone ranging in frequency from 900 to 1700 Hz.
Audio filters or detectors of the type normally used in receivers for this purpose have a band-pass of 8 to 10% of its center frequency or .+-.130 Hz, in the example given. It is therefore clear that a filter with a band-pass this narrow might not detect a 1300 Hz tone at all times, even assuming the permissable frequency variation in the transmitter.